We present a massively parallel algorithm for 3D acoustic full waveform inversion together with an application to OBC data from the Valhall field. To achieve a computational efficiency and a flexible algorithm, we design a process, which can combine various forward modelling engines (such as finite-difference or finite-element methods) in time or frequency domains and an inversion core formulated in the frequency domain. Moreover our algorithm contains two nested levels of parallelism : source distribution and domain decomposition are implemented for the optimization of the performances of the scheme with respect to the computational platform, the dimensions of the model and the acquisition geometry. We present an application of our algorithm to OBC data recorded in the Valhall field. Two overlapping groups of frequencies, [3.5-4] and [4-5] Hz, are inverted successively to build a P-wave velocity model from the hydrophone component. The final FWI model shows several structures, which are consistent with previous studies: channels below the sea-floor, low velocity and fractured zones in depths, probably related to accumulation of gas, and deep reflectors below the reservoir level. Comparison between a sonic log of vertical velocity and FWI log possibly reveals the footprint of the anisotropy.


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